1. Field of the Invention
The present invention relates to a process for producing a sinterable crystalline aluminum nitride powder, and more particularly, to a process for producing aluminum nitride powder containing an amount of impurities low enough that it can be used for producing aluminum nitride substrates for semiconductors and the like.
2. Description of the Prior Art
In the field of microelectronics, large scale integration and high output are recently more and more in demand and, therefore, conventional alumina substrates have become unsatisfactory as a heat releasing substrate for such semiconductor devices.
As a result, an aluminum nitride substrate draws attention as a new heat releasing substrate because of its high thermal conductivity, high heat resistance and high insulating properties.
In connection with the foregoing situation, there are various known processes for producing an aluminum nitride powder. They are, for example, (1) a process comprising heating metallic aluminum in a nitrogen or ammonia atmosphere, (2) a process comprising mixing alumina powder and carbon powder and heating the mixture in a nitrogen or ammonia atmosphere, (3) a process comprising applying a heat treatment to a reaction product of organic aluminum and amines, and (4) a process comprising reacting ammonia with aluminum chloride or aluminum bromide gas or a mixture gas thereof in a vapor phase.
However, these processes have drawbacks. With respect to process (1), since the melting point of aluminum is near to the nitriding temperature, the product solidifies or a highly pure product can not be obtained because impurities in the starting material can not be removed. With respect to process (2), the particle size of the alumina starting material and the impurities therein are brought into the product without change so that highly pure starting materials of a uniform particle size are required. With respect to process (3), a lot of residual carbon remains in the product and its removal is difficult. As for process (4), the by-product, a hydrogen halide gas, disadvantageously corrodes the apparatus and the waste gas treatment and the like are also problems.
A new process for producing crystallized aluminum nitride has been recently proposed which comprises mixing an alkyl aluminum and ammonia at a molar ratio of flow rate of ammonia gas to alkyl aluminum of at least 5 at a temperature not higher than 200.degree. C. and carrying out the vapor reaction at 600.degree.-1300.degree. C. to produce amorphous aluminum nitride powder followed by calcining at 1400.degree. C. or higher for crystallization thereof (Japanese Patent Application Laid-open Sho 63-60102). This process is carried out by a vapor phase reaction and therefore advantageously gives a powder with less aggregation and a relatively good sintering property as compared with a liquid phase reaction. However, as a result of the present inventor's investigation, the following problems arise when using this process for an industrial production.
Firstly, when ammonia gas and alkyl aluminum are mixed at a temperature not higher than 200.degree. C., the resulting complex does not reach the decomposition temperature and a white sticky matter is formed which clogs nozzles and reactor pipes. In other words, if the reacting gases are mixed under these conditions, such clogging makes long run continuous operation difficult. If both starting materials are mixed at a higher temperature, self-decomposition of alkyl aluminum causes clogging of the feed line to a significant extent. Further, although the amorphous powder is calcined at a temperature not lower than 1400.degree. C. for crystallization thereof, it is particularly difficult in large scale industrial production to reduce the carbon content in the product to less than 0.1%. The reason for this is not completely clear, but it appears that on scaling up, the reaction does not occur uniformly due to turbulence of the gas stream in the reactor pipe and as a result the residual carbon content in the amorphous powder increases.
A method for synthesizing aluminum nitride by a vapor phase reaction of organic aluminum compounds with ammonia gas is reported, for example, by Manasevit et al., Journal of the Electrochemical Society, Vol. 118, No. 11, pp. 1864-1868 (1971), in which trimethyl aluminum and ammonia are reacted in a vapor phase to grow aluminum nitride crystals on a heated substrate. However, the carbon content of the aluminum nitride thus obtained is as high as 1% by weight and the purpose is not the production of aluminum nitride powder, but vapor-deposition thereof on a substrate.
In general, as disclosed, for example, in Yogyo Kyokai--shi, Vol. 12, p. 95, 1987, it is preferable that both the oxygen and carbon contents of aluminum nitride powder used as fine ceramics raw materials are low. Although the sintering property is degraded when oxygen is not present in the powder at all, nevertheless, in general the oxygen content is preferably less than 1% by weight, more preferably less than 0.6% by weight and the carbon content is preferably less than 0.5%, more preferably less than 0.1% by weight. (Hereinafter, oxygen and carbon contents are in terms of % by weight unless otherwise noted.)
With respect to particle size, the aluminum nitride powder is preferably a fine powder having a particle size of not larger than 0.1.mu.. However, the smaller the particle size, the larger the surface activity, and therefore, it is difficult to lower the oxygen content to less than 1% due to the adsorbed oxygen even if the powder is handled as carefully as possible in an inert gas.
In any event, the aluminum nitride powder produced by the above-mentioned methods contains a fairly large amount of oxygen and carbon as impurities in differing proportions depending upon the reaction mode and crystallization conditions. Therefore, the sintered body prepared from the aluminum nitride powder does not have sufficiently high heat resistance, high insulating property, in particular, high thermal conductivity and in addition, a method for removing the impurities is not yet available and its development is needed.